This invention relates to apparatus for removing impurities from liquid metal and more particularly for removing impurities from liquid metal in a fast breeder reactor.
For successful operation of high temperature circulating liquid metal systems, whether a liquid metal fast breeder reactor, or a smaller developmental facility, it is accepted that the liquid metal must be of a high purity particularly with respect to oxygen. To maintain the liquid metal in a state of high purity, the cold trap concept is generally used. Cold trapping action depends upon the decreasing solubility of impurities with decreasing temperature. In a typical cold trap, a liquid metal stream is cooled as it passes through a subsidiary system which includes a vessel where the impurities are precipitated and held as solid phases. There are at least two properties that are desirable in cold traps: (1) the trap should remove impurities at or close to the maximum rate theoretically possible; and (2) the trap should be capable of using a high proportion of its volume as storage capacity for impurities.
In a conventional cold trap, the liquid metal enters the top of the trap from a heat exchanger and flows downwardly through the outer annulus or "downcomer". The flow direction reverses at the bottom of the cold trap and the liquid metal enters the end of a cylindrical volume packed with mesh. The liquid metal then flows upwardly through the cylinder and out through the end of the cylinder to an exit pipe. In general, nucleation of precipitate occurs largely on cold surfaces of the cold trap. This is sometimes referred to as heterogeneous nucleation. Once the nucleation has thus begun, the nuclei then can grow. The driving force for this process is the excess solute concentration above saturation solubility at the given temperature, otherwise known as "supersaturation". The typical cold traps are externally cooled with the downcomer region acting as a heat exchanger. While there is usually not a large temperature difference along the cold trap, nevertheless, the bottom of the cold trap is normally the coldest region. Hence, nucleation and growth of precipitates can occur on the outer vessel wall and over the cold trap bottom as has been found in examinations of used cold traps. In the conventional designs, the extended surface of the inlet section of mesh is also located in the coldest region. Thus, nucleation and growth of nuclei occur in this region. However, two factors mitigate against the extension of nucleation and growth in the downstream portion of the mesh. First, growth of the nuclei in the mesh entrance occurs as the impure liquid metal stream passes through, but this reduces the degree of supersaturation in the stream. Therefore, further nucleation on surfaces downstream becomes less likely. This is particularly true since nucleation requires a higher degree of supersaturation than does growth of existing nuclei. Secondly, such tendencies are further enhanced by the heat exchanger action of the cold trap downcomer. In the typical cold trap, the fluid stream experiences some reheating as it moves through the mesh because of its location adjacent to the downcomer region wherein the hot liquid stream is passing. Therefore, the upwardly flowing liquid stream experiences some reheating as it moves through the mesh thereby reducing still further the degree of supersaturation. For these reasons, crystal growth is generally confined to a small proportion of the mesh volume at the inlet end of the cold trap. Under these circumstances, the flow path eventually becomes restricted and the cold trap is plugged even though only a small proportion of the cold trap volume is filled with impurities.
Another disadvantage with the conventional cold trap design is the tendency for the cold trap exit line to become plugged. As previously described, in the conventional cold trap design, the impurity removal mechanisms generally occur at the entrance to the mesh area with impurity removal mechanisms not being effective near the end of the mesh area. Since some degree of supersaturation generally remains as the fluid flow nears the exit of the cold trap and since nucleation is enhanced by increased flow turbulence, the sudden increase in turbulence of the fluid stream as the fluid stream enters a narrow exit line may be sufficient to induce heterogeneous nucleation from the residual supersaturation. Therefore, nucleation can occur in the exit line. Since a slight degree of supersaturation exists at this point, the new crystals can proceed to grow and the line can be plugged relatively quickly.
Therefore, what is needed is a liquid metal cold trap wherein a greater portion of the volume of the cold trap can be utilized for removing impurities from the liquid metal.